Human serum albumin is a protein consisting of 585 amino acids which does not contain associated glycoside residues and has a molecular weight of the order of 66,000 daltons.
Genetically, human serum albumin is encoded in man by two codominant autosomal allelic genes. The genes for human serum albumin are notoriously polymorphic, and at least twenty-four variants of serum albumin are known, differentiated by their electrophoretic behaviour (Shell and Blumberg, "The genetics of human serum albumin", in "Albumin Structure, Function and Uses", Rosenoer, Oratz and Rothschild eds., Pergamon Press, 1977).
Serum albumin is synthesized in the hepatocytes, and then excreted into the serum in which it constitutes the most abundant protein, with mean concentrations of the order of 4g/100 ml of serum. It performs a major physiological role in the maintenance of the osmotic pressure of the plasma, and thus contributes to the stability of the balance between the internal (cellular) environment and the external (circulating) environment, which balance provides, among other functions, for the maintenance of a level of cell hydration which is compatible with the normal physiological functioning of the body.
Human serum albumin also performs a role in the transport of "natural" hydrophobic molecules (steroids and bile salts, for example) and drug molecules to their sites of action.
This explains why human serum albumin is used both in the therapy of blood volume disorders, for example posthaemorrhagic acute hypovolaemia or extensive burns, and in supportive therapy in so-called volume expansion solutions in general surgery, and in the treatment of dehydration states (for example nephrotic syndromes), all these uses demanding the supply of considerable amounts of serum albumin (several tens of grammes per day per patient).
Human serum albumin is at present extracted from serum by techniques derived from that of E.J. Cohn et al., J. Am. Chem. Soc. (1946), 68, p. 459 et seq., or from placenta by the technique of J. Liautaud et al., 13th Internat. Congress of IABS, Budapest; A: Purification of Proteins, Development of Biological Standard (1973) Karger, ed., Bale, 27, p. 107 et seq. These sources, which hardly meet the requirements of the world market, suffer from several defects, inter alia their uncertain nature. Moreover, they are not devoid of the risk of contamination (hepatitis, for example, and more recently acquired immunodeficiency syndrome), and this would have dramatic consequences when the protein was used in therapy.
In vitro genetic recombination techniques now offer the possibility of making a micro-organism, for example the Escherichia coli bacterium, synthesize any protein or any polypeptide and, in theory, doing this in unlimited quantities (see for example F. Gros et al., Sciences de la Vie et Societe, Documentation Francaise ed., 1979).
Since the classical experiments of F. Jacob et al., it is known that DNA contains, on the one hand a group of so-called "structural" genes, that is to say genes which code for a given protein, and on the other hand so-called "regulator" genes, that is to say genes capable of modulating the expression of the structural genes, the combination of the two types forming an entity known as an "operon".
Research in molecular biology and the development of DNA sequencing techniques [F. Sanger and A.R. Coulson, J. Mol. Biol. (1975), 94, p. 441 et seq., A.M. Maxam and W. Gilbert, Proc. Natl. Acad. Sci. (USA) (1977), 74, p. 560 et seq.] have made it possible to specify the organization of the operon as it had been conceived by F. Jacob and J. Monod [F. Jacob and J. Monod, Cold Spring Harbor Symp. Quant. Biol. (1961), 26, p. 193 et seq.; F. Jacob and J. Monod, J. Mol. Biol. (1961), 26 p. 318 et seq.], and to identify the special features of the primary structure of the two types of gene.
Thus, all structural genes are enclosed by a so-called "translation initiation" codon (ATG) and a "stop" codon. The function of the initiation codon is to bind a transfer RNA bearing a formylmethionine. The protein chain will elongate from this formylmethionine by successive attachment of amino acids encoded by the structural gene; the "stop" codon will finally cause the elongation to stop and bring about the release of the newly formed protein.
As regards the regulating genes (promoters, repressors), a promoter, for example, being defined as a DNA fragment to which RNA polymerase is bound, it has been possible to identify the most highly conserved sequences [D. Pribnow, Proc. Natl. Acad. Sci. (USA) (1975), 72, p. 784 et seq.]; similarly, it has been possible to define the most highly conserved DNA sequences at the level of the ribosome binding sites (RBS) [J. Shine and L. Dalgarno, Nature (1975), 254, p. 34 et seq.], which sites perform a role in the translation of the transcribed RNA to protein.
Thus, the bacterial regulator genes can hence be defined by their functional properties and also by their primary sequence, and in vitro genetic recombination techniques turn this to good account to place any structural gene under their control, this being possible as a result of the existence of "restriction enzymes" which cut the DNA at specific points [H.0. Smith and K.W. Wilcox, J. Mol. Biol. (1970), 51, p. 379 et seq., M. Meselson and R. Yuan, Nature (1968), 217, p. 1110 et seq., R.J. Roberts, Nucleic Acids Res. (1982), 1, p. 135 et seq.].
The techniques used, which are in other respects known, employ the concerted use of these enzymes to cut the DNA at predetermined points, and enzymes known as "ligases" to link the fragments together [P.E. Loban and A.D. Kaiser, J. Mol. Biol. (1973), 78, p. 453 et seq.] The assembly is carried by "vectors" (plasmids or bacteriophages) capable of being introduced into a bacterium such as E. coli by processes which are in other respects known, and of being maintained there during the growth of the host bacterium [M. Mandel and A. Higa, J. Mol. Biol. (1970), 53, p. 154 et seq.].